The two haploid cell types of the heterothallic yeast Saccharomyces cerevisiae, if cultured separately, proliferate by mitotic cell division. However, when diploid cells of opposite mating type are mixed, specific agglutination and cell fusion occurs producing zygotes which can grow by budding off diploid daughter cells. Thus, although every haploid yeast cell has the potential for mating, this potential is expressed only in the presence of and with cells of the opposite mating type. The mating process represents therefore a simple developmental change triggered by specific cell-cell interactions. The purpose of this investigation is to define at the molecular level the signals that pass between the two haploid cell types and the mechanisms by which this information leads to a decision of the cells to conjugate rather than to continue their individual vegetative multiplication. These studies include: purification and chemical characterization of the specific diffusible substances, alpha-factor and a-factor, released by the two haploid cell types; identification and isolation of the yeast cell components that interact with alpha- and a-factor; identification and isolation of the surface molecules which permit the specific adhesion and eventual fusion of the two haploid cells; and, selection and examination of mutants defective in various stages of the mating process to aid biochemical analysis of the cellular components involved in the conjugation response and its control.